M.S.
I was having the same problem with mine. I got her her own toothbrush to hold. So now when she opens her mouth to put her toothbrush in I put the one I am holding in. It is a mouthful but it gets the job done. While I do one side she is "brushing" the other. I put toothpaste on both brushes so at least hers is doing some good. I think she has gotten quite good at controlling her own toothbrush doing it that way. I hold her in front of the mirror so she can see what I am doing and she copies me. Hope this helps!!!